Many industrial plants contain machines with hydraulic actuators, and with oil reservoirs where the hydraulic fluid, or oil, is stored. The oil becomes contaminated with dirt during use, and becomes contaminated with water especially due to condensation when the machine is not used at night and on weekends. One approach to cleaning the oil is to provide a dedicated cleaning system for the machine, such as a centrifugal pump which separates water and dirt from oil. U.S. Pat. No. 4,431,540 by Budzich shows a system of this type. However, this can add substantially to the space requirements and cost of the machine. An oil cleaning system for an industrial plant, which minimized the space requirements and cost for oil cleaning and which enabled very extensive cleaning when necessary, would be of considerable value.